1. Field of the Invention
This invention relates to an optical memory medium with which information can be optically recorded and reproduced or erased. More particularly, it relates to an improvement for an a preformat of optical memory medium.
2. Description of the Prior Art
In recent years, the development of information processing systems large capacity has created a demand for inexpensive memories. To meet this demand, various optical memory systems have been developed. Among such optical memory systems, three kinds of systems have been used.
The first one is a system which only reproduces information. In this system, fine pit patterns are formed in a row on a recording element so that information can be reproduced by utilizing the diffraction phenomenon of an incident laser beam in the pit patterns.
The second system is of a so-called write once type in which a laser beam is applied to a recording medium to form portions. The reflectivity of the medium is changed, and the change in the reflectivity is detected by applying a laser beam, to read the recorded information. In this system, hence, information can be additionally recorded by a user.
The third system employs a magneto-optic disk in which information can be recorded, and erased by a user (i.e., information is rewritable). A magneto-optic disk used in this system has a magnetic thin film with an axis of easy magnetization perpendicular to the surface of the film. A portion of the magnetic thin film irradiated by a laser beam is heated to decrease the coercive force of the film. In the irradiated portion, the magnetic domains are oriented in accordance with the direction of the external magnetic field applied to the film thereby the recording and erasing the information. Information is reproduced based on the magneto-optic effect while a weak laser beam is irradiated to the portion in which information has been recorded.
In an optical disk used in such a system, "recording tracks", having a width of about 1 .mu.m store the information. If a laser spot is deviated from the objective recording track or applied to a recording track other than the objective track, information cannot be correctly recorded or reproduced. When recording or reproducing information, therefore, the position of a final focused laser spot should be precisely controlled so as not to deviate from the objective recording track, thereby necessitating a servo control system which uses guiding tracks of any kind.
An optical disk (e.g., a so-called compact disk) used in the aforementioned optical system which only reproduces of information includes row-formed pit patterns corresponding to information to be recorded and which are formed in the manufacturing process of the disk. In such an optical disk, therefore, the rows (recording tracks) composed of the pit patterns can be used as guiding tracks so that a laser spot can be servo-controlled to trace one of the rows.
In an optical disk of a write once type or rewritable type, information is recorded into regions no information has been prerecorded. In order to accurately guide the laser spot to a predetermined position (i.e., a predetermined recording track) of the disk, guide tracks and addresses indicating a track number or address are preformed on a glass or resin substrate of the disk in the manufacturing process of the disk. Hereinafter, the term "optical disk" is used to indicate an optical disk in which information can be recorded after the production of the disk, such as those of a write once type or a rewritable type, including a magneto-optic disk.
Generally, guide tracks in the form of grooves are intermittently or continuously formed along in a circumferential direction on a substrate of an optical disk, and address information is recorded in the form of pits along the grooves. Some methods of preforming the guide tracks and pits have been proposed. For example, a method in which an Ni stamper is employed to transcribe the grooves and pits onto a resin substrate made of acrylic resin or polycarbonate resin by an injection molding technique, and a method in which UV-setting resin is interposed between a stamper having grooves and pits and a substrate made of glass or acrylic resin, and the interposed resin is then exposed to UV rays (this method called "the 2P method"). Another proposed method uses a glass substrate, on which a laser beam or UV rays are selectively irradiated through a photo mask onto a photoresist layer, and then an etching is done to form simultaneously grooves and pits.
The guide tracks are V-shaped, U-shaped or .OMEGA.-shaped in cross sectional view, and are formed into a spiral or concentric circles on the optical disks. Each of the guide tracks is provided with information of a track number or address, called prepit. The prepits and the guide tracks are named, together, to be a preformat. Preformats include a groove-type preformat (FIG. 6) in which data bits 3 are disposed on guide tracks 1 and prepits 2 are disposed in an area, in which there is no guide track 1, extending from one end of a guide track section to the other end thereof. A land-type preformat (FIG. 7) has data bits 3 and prepits 2 disposed on a land positioned between the adjacent guide tracks 1. If the optical disk is of a magneto-optic type, the data bits 3 represent portions in which the direction of magnetization is reversed to store information.
The shape of each of the guide tracks 1 is determined so that tracking error signals can be distinctly obtained from the guide tracks 1 when the the optical beam position deviates. The shape of each of the prepits 2 is determined so that a signal that identifies the track number can be distinctly output.
Accordingly, there is usually a difference between the optimum size of the prepits 2 and the optimum size of the guide tracks 1.
With the above-mentioned groove-type preformat, the position of an optical disk on which the prepits 2 are formed is a position on which the guide track 1 is not formed, so that the tracking error signals from the guide tracks 1 cannot be output when the laser beam is positioned in the said prepits. On the other hand, the above-mentioned land-type preformat is designed so that the guide tracks 1 pass along the position on which the prepits 2 are formed. Accordingly, the land-type format is disadvantageous in that the prepits 2 influence negatively the tracking error signals from the guide track 1 although the tracking error signals from the guide track 1 can be output.
To overcome the above-mentioned problem, the depth of the groove of each of the guide tracks is differentiated from the depth of the groove of each of the prepits 2. For example, the depth of the grooves that constitute the prepits 2 are set to be 1/4 wavelength of laser light, which causes difficulties in the production of the preformat.